Unraveling the Bimodality of Polypropylene Film Grades Using Rheological Shear and Elongational Measurements: Inconsistent Results of Gel Permeation Chromatography

As the main parameter to describe the molecular structure of a linear polymer, molecular weight distribution of five different polypropylene grades was evaluated through measurements of gel permeation chromatography and rheology experiments. From their dynamic shear data, the relaxation spectra, h(τ), were calculated from which molecular weight distribution was estimated using molecular viscoelastic theories and then compared with the gel permeation chromatography results. Generally good agreement, yet with narrower molecular weight distribution rheology curves was found. Exception was observed for one grade for which rheological data predicted bimodal distribution curve comprising a small shoulder of high molecular weight, not seen in the corresponding gel permeation chromatography curve. Additionally, a higher value of the generalized mixing parameter (i.e. β) was found for this bimodal grade which was attributed to an increase in the number of entanglements per chain and better network connectivity. Furthermore, measurements of the transient extensional viscosity also revealed enhanced strain hardening and melt strength for this particular grade. The results showed that small amount of high molecular weight fraction of linear chains could drastically alter the processing behavior of polypropylene film grades.     

Interaction of coinage metal clusters with chalcogen dihydrides

The interaction of chalcogen dihydrides (H 2E; E = O, S, and Se) with small coinage metal clusters (M n ; M = Cu, Ag, and Au, n = 3 and 4) is studied based on density functional theory, with a focus on the nature of chalcogen-metal bonds. A newly developed pseudopotential-based correlation-consistent basis set is used for metal clusters together with the 6-311++G** basis set for the remaining atoms. Geometrical data identified that no significant deviation has been observed for molecules before and after complexation. For these three metals, binding energy calculations indicate that gold has the highest and silver has the lowest affinities for interaction with H 2E. In comparison with gold and copper, complexation between silver and chalcogen dihydrides is significantly weaker. It is found that interaction of H 2E molecules with the coinage metals have the order of H 2Se > H 2S > H 2O. Therefore, in agreement with experimental works, our calculations confirm that the gold-selenium bond is the most stable. The nature of M-E bonds is also interpreted by means of the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. According to the QTAIM results, the bonds are found to be partially ionic and partially covalent. Natural resonance theory (NRT) is used to calculate natural bond order and bond polarity. The NRT result indicates that the percentage of polarity of M-E bonds is affected by coinage metals.     

Pd-catalyzed aryl amination mediated by well defined, N-heterocyclic carbene (NHC)-Pd precatalysts, PEPPSI

Pd-N-heterocyclic carbene (NHC)-catalyzed Buchwald-Hartwig amination protocols mediated by Pd-PEPPSI precatalysts is described. These protocols provide access to a range of hindered and functionalized drug-like aryl amines in high yield with both electron-deficient and electron-rich aryl- and heteroaryl chlorides and bromides. Variations in solvent polarity, base and temperature are tolerated, enhancing the scope and utility of this protocol. A mechanistic rationalization for base strength (pKb) requirements is also provided.     

A user-friendly, all-purpose Pd-NHC (NHC=N-heterocyclic carbene) precatalyst for the negishi reaction: a step towards a universal cross-coupling catalyst

We have developed the first user-friendly Negishi protocol capable of routinely cross-coupling all combinations of alkyl and aryl centers. The use of an easily synthesized, air stable, highly active, well-defined precatalyst PEPPSI-IPr (1; PEPPSI=pyridine-enhanced precatalyst preparation, stabilization and initiation; IPr=diisopropylphenylimidazolium derivative) substantially increases the scope, reliability, and ease-of-use of the Negishi reaction. All organohalides and routinely used pseudohalides were excellent coupling partners, with the use of chlorides, bromides, iodides, triflates, tosylates, and mesylates resulting in high yield of the coupled product. Furthermore, all reactions were performed by using general laboratory techniques, with no glove-box necessary as the precatalyst was weighed and stored in air. Utilization of this methodology allowed for the easy synthesis of an assortment of sterically encumbered biaryls and druglike heteroaromatics, demonstrating the value of the PEPPSI-IPr system. Furthermore, this is also the first time Pd-NHC (NHC=N-heterocyclic carbene) methodology has surpassed the related phosphine-ligated Negishi processes both in activity and use.     

Easily prepared air- and moisture-stable Pd-NHC (NHC=N-heterocyclic carbene) complexes: a reliable, user-friendly, highly active palladium precatalyst for the Suzuki-Miyaura reaction

The synthesis of NHC-PdCl(2)-3-chloropyridine (NHC=N-heterocyclic carbene) complexes from readily available starting materials in air is described. The 2,6-diisopropylphenyl derivative was found to be highly catalytically active in alkyl-alkyl Suzuki and Negishi cross-coupling reactions. The synthesis, ease-of-use, and activity of this complex are substantial improvements over in situ catalyst generation and all current Pd-NHC complexes. The utilization of complex 4 led to the development of a reliable, easily employed Suzuki-Miyama protocol. Employing various reaction conditions allowed a large array of hindered biaryl and drug-like heteroaromatic compounds to be synthesized without difficulty.     

Observation and assignment of phonon combination bands in the far infrared dielectric response of GaAs

A determination by dispersive Fourier transform spectroscopy of the frequency dependence of the anharmonic self-energy of the q??O transverse optic phonon in GaAs at 300 and 6 K is presented. The measured self-energy function is compared with the two-phonon density of states function calculated from an 11-parameter rigid ion model, and the model is found to account satisfactorily for all features in the measured spectrum as phonon combination bands.     

First observation in the gas phase of the ultrafast electronic relaxation pathways of the S(2) states of heme and hemin

The time evolution of electronically excited heme (iron II protoporphyrin IX, [Fe(II) PP]) and its associated salt hemin (iron III protoporphyrin IX chloride, [Fe(III) PP-Cl]), has been investigated for the first time in the gas phase by femtosecond pump-probe spectroscopy. The porphyrins were excited at 400 nm in the S(2) state (Soret band) and their relaxation dynamics was probed by multiphoton ionization at 800 nm. This time evolution was compared with that of the excited state of zinc protoporphyrin IX [Zn PP] whose S(2) excited state likely decays to the long lived S(1) state through a conical intersection, in less than 100 fs. Instead, for [Fe(II) PP] and [Fe(III) PP-Cl], the key relaxation step from S(2) is interpreted as an ultrafast charge transfer from the porphyrin excited orbital π* to a vacant d orbital on the iron atom (ligand to metal charge transfer, LMCT). This intermediate LMCT state then relaxes to the ground state within 250 fs. Through this work a new, serendipitous, preparation step was found for Fe(II) porphyrins, in the gas phase.     

Density Functional Theory Investigation of the Alkyl–Alkyl Negishi Cross‐Coupling Reaction Catalyzed by N‐Heterocyclic Carbene (NHC)–Pd Complexes

Why bigger is better : A “steric wall” created by the N‐(2,6‐diisopropylphenyl) substituent on the bulky NHC ligand IPr (1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) guides the reactants to and from the Pd center through weak, fleeting (IPr)H–Pd interactions that help the oxidative addition intermediate escape “the anti‐trap”. The alternative “side” approach leads to transmetalation (the rate‐limiting step) for which a novel Pd–Zn interaction was identified.

     

Synthesis,characterization, anticancer activity,thermal and electrochemical studies of some novel uranyl Schiff base complexes

Some tetradentate N₂O₂ Schiff base ligands, such as N,N′-bis(naphtalidene)-1,2-phenylenediamine, N,N′-bis(naphtalidene)-4-methyl-1,2-phenylenediamine, N,N′-bis(naphtalidene)-4-chloro-1,2-phenylenediamine, N,N′-bis(naphtalidene)-4-nitro-1,2-phenylenediamine, N,N′-bis(naphtalidene)-4-carboxyl-1,2-phenylenediamine, and their uranyl complexes were synthesized and characterized by ¹H NMR, IR, UV–Vis spectroscopy, TG (thermogravimetry), and elemental analysis (C.H.N.). Thermogravimetric analysis shows that uranyl complexes have very different thermal stabilities. This method is used also to establish that only one solvent molecule is coordinated to the central uranium ion and this solvent molecule does not coordinate strongly and is removed easier than the tetradentate ligand and also trans oxides. The electrochemical properties of the uranyl complexes were investigated by cyclic voltammetry. Electrochemistry of these complexes showed a quasireversible redox reaction without any successive reactions. Also, the kinetic parameters of thermal decomposition were calculated using Coats–Redfern equation. According to Coats–Redfern plots the kinetics of thermal decomposition of the studied complexes is first-order in all stages. Anticancer activity of the uranyl Schiff base complexes against cancer cell lines (Jurkat) was studied and determined by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliumbromide) assay.     

Synthesis,structural characterization,density functional theory calculations and biological evaluation of a new organotin(IV) complex containing N‐isonicotinyl‐N',N″‐diaryl phosphorictriamide as N‐donor ligand

A new diorganotin(IV) complex with the formula SnCl₂(CH₃)₂L₂ ( C_1a ), L = 4‐NC₅H₄CONHPO(NCH₃CH₂C₆H₅)₂, was synthesized and characterized using ¹H NMR, ¹³C NMR, ³¹P NMR, ¹¹⁹Sn NMR and infrared spectroscopies. The molecular structure of C_1a was determined using X‐ray crystallography, revealing that C_1a contains hexa‐coordinated Sn(IV) centres with trans‐configuration of donor atoms around them. Each Sn(IV) atom is positioned in the centre of inversion of an octahedron. C_1a forms one‐dimensional chains via two equal intermolecular P?O…H? N hydrogen bonds. These hydrogen bonds produce centrosymmetric rings as a supramolecular hydrogen‐bonded pattern. In order to compare the relative stability of C_1a (with N‐ligated configuration) and its possible O‐ligated isomer, C_1b , density functional theory calculations were performed, the results showing a preference of C_1a over C_1b from an energy point of view. Also, natural bond orbital analysis was carried out to obtain detailed information on the electronic features of the optimized structures. The theoretical results show that intermolecular hydrogen bonding in the crystal structure has a significant role in the stabilization of C_1a , and Sn(IV) interacts more strongly with the N_py atom than the P?O functional group. Furthermore, the free ligand and its complex were tested against three human cancer cell lines, i.e. human cervical carcinoma (HeLa), human prostate cancer (PC‐3) and human breast adenocarcinoma cancer (MCF‐7). C_1a displays moderate to good cytotoxicity towards all three cancer cell lines. Moreover, antibacterial tests were carried out using the disc‐diffusion method, in which C_1a shows high activity against selected Gram‐negative and Gram‐positive bacteria. Copyright © 2015 John Wiley & Sons, Ltd.